2008 Annual Science Report
University of Wisconsin Reporting | JUL 2007 – JUN 2008
New Frontiers in Micro-Analysis of Isotopic Compositions of Natural Materials: Development of O, Si, and Li Isotopes
The isotope ratios of oxygen and silicon are a sensitive monitor of sedimentary and hydrothermal processes for deposition of banded iron formation. Our focus on banded iron formations reflects the importance these unusual units have in biogeochemical cycling in the early Earth. In particular, we are examining the deposition of microlaminated sections of the Dales Gorge member of the Brockman Iron Formation from the Hamersley basin, which have alternatively been interpreted to represent annual varves in chemical precipitates from seawater, or variations in a sub-surface hydrothermal system. These conflicting models have relevance for interpreting the role of microbial life in precipitation of the Fe-oxides. In addition, δ18O and δ34S values from coexisting minerals can be used to estimate the temperature of the Archean ocean, or of the hydrothermal system. δ7Li and δ18O values in zircons allow these tests to be applied to magmas that may have assimilated sedimentary materials, and, in the case of Jack Hills samples from SW Australia, provide a record of the earliest Earth (4.4 to 4.0 Ga), before the formation of all known rocks.
Using the University of Wisconsin’s new CAMECA ims-1280 ion microprobe, we have developed the ability to make analyses of: δ18O and δ34S from spots that are 1 to 10 μm in diameter with precision as good as 0.2‰ (2sd); δ30Si in quartz with precision of 0.2‰; and δ7Li in zircons that contain as little as 1 ppm Li. Isotope standards have been developed for O and Si in quartz, S in pyrite, and Li in zircon. We are now working to develop standards for O in magnetite and S in sphalerite. Detailed petrography by optics and SEM-BSE is underway on BIFs samples from Dales Gorge, Kuruman (South Africa), Isua, and Biwabik/Gunflint; work is also planned for the 3.5 Ga Apex and Strelley Pool cherts.
PROJECT INVESTIGATORS:John Valley
PROJECT MEMBERS:Noriko Kita
RELATED OBJECTIVES:Objective 4.1
Earth's early biosphere
Biosignatures to be sought in Solar System materials